1) Need to include GL/glext.h on W32 to provide necessary macros
2) W32 macros that pre-processor has by default are different for different
toolchains. Borland uses __WIN32__, everyone else has _WIN32, so check both.
Side-note: glext.h is not supplied by mingw-w64 at the moment, but can be
downloaded from http://www.opengl.org/registry/
https://bugzilla.gnome.org/show_bug.cgi?id=703364
# else
# include <GL/glu.h>
# include <GL/gl.h>
-# if __WIN32__
+# if __WIN32__ || _WIN32
# include <GL/glext.h>
# endif
# endif